Direct Visualization of Supramolecular Binding and Separation of Light Hydrocarbons in MFM-300(In)

The purification of light olefins is one of the most important chemical separations globally and consumes large amounts of energy. Porous materials have the capability to improve the efficiency of this process by acting as solid, regenerable adsorbents. However, to develop translational systems, the underlying mechanisms of adsorption in porous materials must be fully understood. Herein, we report the adsorption and dynamic separation of C2 and C3 hydrocarbons in the metal–organic framework MFM-300(In), which exhibits excellent performance in the separation of mixtures of ethane/ethylene and propyne/propylene. Unusually selective adsorption of ethane over ethylene at low pressure is observed, resulting in selective retention of ethane from a mixture of ethylene/ethane, thus demonstrating its potential for a one-step purification of ethylene (purity > 99.9%). In situ neutron powder diffraction and inelastic neutron scattering reveal the preferred adsorption domains and host–guest binding dynamics of adsorption of C2 and C3 hydrocarbons in MFM-300(In).

exchanged samples were loaded into the system and degassed at 120 °C and 1 × 10 −6 mbar for 20 h to give a dry, desolvated material of typical mass ca. 50 mg. Ultra-pure research grade (99.99 %) gases were purchased from Air Liquide or BOC and used as received. C2H2 was purified by dual-stage cold trap systems operated at 195 K (dry ice) and an activated carbon filter before introduction to the IGA system. Dynamic breakthrough experiments were conducted on a Hiden Isochema IGA-003 with ABR attachments and a Hiden Analytical mass spectrometer by using a fixed-bed tube packed with 750 mg of MFM-300(In) powder. The sample was heated at 120 ºC under a flow of dry He for 12h for activation, and then cooled to room temperature

Thermogravimetric Analysis
The as-synthesised, acetone exchanged and activated MFM-300(In) were heated from room temperature to

Analysis and Derivation of the Isosteric Heats of Adsorption
To estimate the isosteric enthalpies (ΔH) for adsorption of C2H2, C2H4, C2H6, C3H4, C3H6 and C3H8 isotherms between 273-308 K were fitted to the Van t' Hoff equation; where p is pressure in Pa, T is the temperature, and R is the ideal gas constant. All linear fittings show R 2 above 0.99 indicating the consistency of the isotherm data and of the fitting.

Calculation of IAST selectivity for gas adsorption.
To estimate the selectivity observed for each substrate isotherm data at 293 K were fitted using the dual-site Langmuir-Freundlich (DSLF) model (equation 2).
where is the pressure of the bulk gas at equilibrium with the adsorbed phase, is the maximum adsorption amount, is the the affinity constant and is the deviation from the simple Langmuir equation. Using this fitting, the IAST selectivity can be calculated by equation 3.

Calculation of dynamic adsorption capacity and productivity
To determine the dynamic adsorption capacity, the uptake of each component (nm) was calculated based on the breakthrough curves by the equation described as follows: where vgas out is the flow rate of the target gas with the unit of mL min −1 ; Vdead is the dead volume of the system (mL); W represents the mass of MFM-300(In) packed in the breakthrough bed (g); t is the retention time for the specific gas (min); P is atmospheric pressure (Kpa); R is Avogadro constant. T is the measurement temperature (K).
The productivity (qm) of C2H4 and C3H6 was determined through the breakthrough amount of C2H4 and C3H6, which is calculated by integration of the breakthrough curves during a period from t1 to t2 during which the gas purity is greater than 99.9%: where vgas out is the flow rate of target gas with the unit of mL min −1 ; Vdead is the dead volume of the system (mL); W represents the mass of MFM-300(In) packed in the breakthrough bed (g);

Neutron Powder Diffraction
Neutron powder diffraction experiments were undertaken at the WISH diffractometer at the ISIS Facility.
MFM-300(In) was loaded into a 6 mm diameter vanadium sample can and outgassed at 1 × 10 −7 mbar and 100 °C for 1 day. The sample was loaded into a liquid helium cryostat and cooled to 7 K for data collection. C2H2, C2H4, C2H6, C3H4, C3H6 and C3H8 gas were introduced by warming the samples to 298 K and the gas dosed volumetrically from a calibrated volume. The gas-loaded sample was then cooled to 7 K over a period of 2 h to ensure good mobility of adsorbed species within the crystalline structure of MFM-300(In) and for a further 30 mins to ensure thermal equilibrium. Rietveld structural refinements were carried out on the NPD data using the TOPAS software package. 21

Inelastic Neutron Scattering Measurement
Inelastic neutron scattering (INS) experiments were undertaken using the TOSCA spectrometer at the ISIS Facility. MFM-300(In) was loaded into an 11 mm diameter vanadium sample can and outgassed at 1 × 10 −7 mbar and 100 °C for 1 day. The sample was loaded into a helium closed cycle refrigerator (CCR) cryostat and cooled to 11 K for data collection. C2H2, C2H4 and C2H6 gas were introduced by warming the sample to 298 K and the gas was dosed volumetrically from a calibrated volume. The gas-loaded sample was then cooled to 7 K over a period of 2 h to ensure good mobility of adsorbed species within the crystalline structure of MFM-300(In). The sample was kept at 7 K for an additional 30 mins before data collection to ensure the thermal equilibrium.